Topaz format is that stuff when it looks kind of like a PDF version of the book where you can change the font size up or down, but otherwise looks like they just dumped a page scan of a paper book onto your Kindle.
It's widely despised because it tends to look ugly and be unstable and crash/freeze the older models of Kindle (and huge filesizes with little reason for them).
Here's an example of one of the Topaz books that I've been tempted by, which happily turns out to be the same price in ePub over at B&N:
A Brief History of the Spanish Language
If you download the sample to your Kindle, you'll see what I mean.